1. Field of the Invention
The present invention relates to a vehicle seat assembly and a heater unit to be used therefor, and in particular, relates to the heating wire layout of a heater to be mounted in the vehicle seat assembly.
2. Description of the Related Art
A vehicle seat assembly comprises a seat cushion having a seating surface on which a person (occupant) sits and a seat back on which the sitting person leans his or her back. The seat cushion and the seat back each comprise a frame, a soft pad surrounding the frame, and a trim cover covering the pad. To increase the comfort of the person, a vehicle seat assembly including a heater is known.
The heater is mounted in both or either one of the seat back and the seat cushion. Specifically, it is placed within the trim cover of the seat back or the seat cushion, and the warming power is controlled by operation of the user. In a typical seat assembly including a heater, a seat heater unit comprising a substrate and a heating wire laid out thereon is fastened within the trim cover.
FIG. 8 is a plan view illustrating an example of a seat heater unit to be mounted in a seat back. The heater unit 8 comprises a flexible sheet-like substrate 81 and a heating wire 82 fastened on the substrate 81. The heater unit 8 is secured between the trim cover and the pad in a seat back. FIG. 8 shows the front side of the heater unit 8, which is the shape of the heater unit 8 when the seat back is viewed from the foreside of a vehicle. The top and the bottom of FIG. 8 correspond to the top and the bottom of the seat assembly, respectively.
In FIG. 8, the upper part above the dashed divider line is placed in the front of the seat back and warms the occupant's body. The lower part below the dashed line is placed between the seat back and the seat cushion, or at the back of the seat back. As shown in FIG. 8, in the upper part above the dashed line, the heating wire 82 has a left-right symmetric form, and is not placed in the middle part which is provided between the right part 821 and the left part 822.
The space G0 in the middle part, where the heating wire 82 does not exist, extends vertically; its location corresponds to the middle in the horizontal direction of the seat back. A heater unit having another configuration is disclosed in Japanese unexamined patent application publication No. 2007-35474 (Arima et al.), for example. The heater unit disclosed in Arima et al. has a space (an area where a heating wire does not exist) between the right side and the left side of the heating wire layout like the heating wire layout shown in FIG. 8.
It is known that if the heater unit 8 is mounted in a seat assembly, the trim cover of the seat back may be creased. That can be found more noticeably in seat assemblies including a particular type of seat back. The inventors have found through their research that the crease is more likely to be formed if the curvature of the front side of the seat back is greater. FIG. 9 is a view illustrating the cross-sectional shape of the seat back pad 91, showing the cross-section of the seat back pad 91 cut perpendicularly to the vertical direction and viewed from the top.
As shown in FIG. 9, the seat back pad 9 (the seat back) in a seat assembly has a side (a front side) 91 curved outward and rearward for supporting the back of the occupant. A heater unit is bonded onto the curved front side 91 of the pad. As explained with reference to FIG. 8, the heating wire 82 does not exist in the space G0 of the heater unit 8. The part of the space G0 is composed of only a flexible substrate 81, so its stiffness is small.
When the heater unit 8 is bonded onto the curved pad, the middle part G0 of the heater unit 8 is likely to be creased. In particular, the small stiffness of the middle part G0 increases the likeliness. On the other hand, even if the heater unit 8 has been successfully bonded to the pad without creases, when the occupant moves with his or her back being in contact with the seat back, the heating wire 82 is gathered toward the middle to deform the heater unit 8, so the middle part G0 with small stiffness is likely to be creased, too. Accordingly, in a vehicle seat assembly with built-in heater, a heater structure is desired that hardly forms creases.